An organic electroluminescence device (hereinafter the term “electroluminescence” is often abbreviated as “EL”) is a self-emission device utilizing the principle that a fluorescent material emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is impressed.
Since C. W. Tang et al. of Eastman Kodak Co. reported a low-voltage driven organic EL device in the form of a stacked type device, studies on organic EL devices wherein organic materials are used as the constituent materials have actively been conducted.
The organic EL device reported by Tang et al. has a stacked structure in which tris(8-hydroxyquinolinol)aluminum is used as an emitting layer and a triphenyldiamine derivative is used as a hole-transporting layer. The advantages of the stack structure are to increase injection efficiency of holes to the emitting layer, to increase generation efficiency of excitons generated by recombination by blocking electrons injected from the cathode, to confine the generated excitons in the emitting layer, and so on.
As the stacked structure of the organic EL device, a two-layered type of a hole-transporting (injecting) layer and an electron-transporting emitting layer, and a three-layered type of a hole-transporting (injecting) layer, an emitting layer and an electron-transporting (injecting) layer are widely known. In such stack structure devices, their device structures and fabrication methods have been contrived to increase recombination efficiency of injected holes and electrons.
As a hole-transporting material used in an organic EL device, an aromatic diamine derivative or an aromatic fused ring diamine derivative has heretofore been known.
However, in order to obtain a sufficient luminance by an organic EL device using these aromatic diamine derivatives as the hole-transporting material, a higher voltage is required to be applied. As a result, problems occur such as a shortened device life or an increased consumption power.
In order to solve these problems, doping a hole-injecting layer of an organic EL device with an electron-accepting compound such as Lewis acid or using an electron-accepting compound singly as an injecting layer has been proposed (Patent Documents 1 to 7, or the like). However, an electron-accepting compound used in Patent Documents 1 to 4 has defects that they are unstable to be handled in the production process of an organic EL device or they cause the life of an organic EL device to be shortened due to insufficiency in stability such as heat resistance when an organic EL device is driven.
In addition, tetrafluoro-tetracyanoquinodimethane (TCNQF4) which is an electron-accepting compound exemplified in Patent Documents 3, 5 to 7 or the like has a small molecular weight and substituted by fluorine. Due to a high sublimation property, it may diffuse within an apparatus when fabricating an organic EL device by vacuum vapor deposition, thereby to contaminate the apparatus or the device.
Patent Document 1: JP-A-2003-031365
Patent Document 2: JP-A-2001-297883
Patent Document 3: JP-A-2000-196140
Patent Document 4: JP-A-H11-251067
Patent Document 5: JP-A-H04-297076
Patent Document 6: JP-T-2004-514257
Patent Document 7: US2005/0255334A1
The invention has been made in view of the above-mentioned problems, and the object thereof is to provide an electron-accepting material which is preferable as a constituting material of an organic EL device.